Optical fibers are used in a variety of optical communications applications, including in so-called “access” and fiber to the premises (FTTx) optical networks. Unfortunately, optical fibers often need to be deployed in such networks in a manner that induces bend losses in optical signals transmitted through the optical fiber. Some circumstances and configurations that can result in tight bend radii, compression of optical fiber, and the like include deploying optical fiber in optical drop cable assemblies, deploying distribution cables with Factory Installed Termination Systems (FITS) and slack loops, small bend radius multi-ports located in cabinets that connect feeder and distribution cables, and jumpers in Network Access Points between distribution and drop cables.
Conventional single-mode optical fibers typically have an SiO2 (silica) glass core region doped with a dopant suitable for raising the refractive index of the core region, with the core surrounded by a cladding glass of pure silica. A typical core region dopant is Ge (e.g., GeO2). An index difference between the core and the cladding is necessary to create a light guide wherein propagating light is generally confined to the core region.
The concentration of GeO2 found in a conventional single-mode doped-core optical fiber may be in excess of 7 weight percent (wt. %). Because the high concentration of dopant is located in the core region of the optical fiber, its attenuation is higher relative to pure silica glass. To overcome the light-absorbing characteristic of a core region containing relatively high concentrations of one or more dopants, pure silica core optical fibers were developed. That is, optical fibers having a core region composed of pure silica have been employed to avoid dopant-based attenuation.
To create the refractive index difference between the core and cladding in a pure silica core optical fiber, one or more refractive-index-modifying dopants are added to the cladding region to reduce the refractive index of the cladding region to a value below the refractive index of the pure silica core region. For example, fluorine (F) can be used as a dopant to decrease the refractive index of the silica glass cladding. The degree to which the refractive index of the cladding of a pure silica core optical fiber is decreased below the refractive index of the core depends upon the optical fiber design and the desired optical fiber parameters.
Doping the cladding region can also affect the viscosity of the cladding glass. That is, when a down-dopant such as F is added to a silica glass cladding region, the viscosity of the cladding region is lowered, resulting in a viscosity mismatch between the pure silica core glass and the doped silica cladding glass. The viscosity mismatch results in residual stress being retained in the fiber during the draw process, and this stress is one cause of transmission loss (attenuation).
To date it has proven difficult to design standards-compliant (e.g., the G.652 standard) optical fibers that have both low loss and low bend loss. The difficulty is due in part to Rayleigh scattering and small-angle scattering attributable to core dopants, the refractive index mismatches between cladding regions, and the viscosity mismatches between the core and cladding regions.